How to have freestyle render change to outline only at a distance
Solution: Use a freestyle line modifier

For your scene I'd recommend using the Distance from Camera modifier on Alpha of the freestyle lines found in the Linestyle panel. The hack is to choose the same value for Range Min and Range Max. This will draw a sharp distinction between the 'close' and the 'far' objects in view.
With two line sets (one for the 'close' objects and the other for the 'far') you can choose for the 'far' line set to only render the outlines.

Here's a simple example

That's the freestyle setup


The scene without any modifiers



And with the modifiers